Complex phase and amplitude modulation schemes have become more popular in optical communications. IQ modulation is a modulation format in which the modulation constellations comprising a given number of constellation points cover the complex IQ plane. For optimum signal-to-noise performance, distinct modulation constellations can be found. For a constellation with three constellation points, the optimum points are the corners of an equilateral triangle. For more points, the optimum constellations are derived from this “simplex” constellation.
An optical IQ modulator typically consists of two parallel Mach-Zehnder amplitude modulators, one each to modulate the I- and Q-value of a symbol. This setup is also referred to as a “nested” Mach-Zehnder modulator. For multi-level modulation formats (more than 4 levels), the drive signals or analogue modulation signals for controlling the modulators are generated by a digital-to-analog converter (DAC). It is desired to reduce the required resolution of these DACs without compromising the noise tolerance of the generated signal. The optical phase between I and Q branches of the nested Mach-Zehnder modulator is 90 degrees.
Such a known optical modulating device 1 is shown in FIG. 1. A digital signal processing device 3 comprising a digital signal processing unit 5, two digital-to-analog converters 7, 9 and two analog amplifiers 11, 13 receives at least one digital transmit signal to be transmitted over an optical path (not shown) connected to an output port 15 of the modulating device 1. The signal processing unit 5 generates two digital modulation signals Smod1 and Smod2 which represent the I-values and the Q-values of the constellation points of a given IQ modulation scheme. The information concerning the modulation scheme and the I- and Q-values of the constellation points, respectively, and the mapping instructions as to how the information included in the at least one digital signal Stx is translated in or mapped to the constellation points and thus translated into the modulated optical signal Stx,mod provided at the output port 15 are implemented via software or hardware in the digital signal processing unit 5 (for example, stored in a dedicated memory or memory area) or are provided to the digital signal processing unit 5 by means of a separate signal (not depicted in FIG. 1). These instructions may include a specific coding of the information to be translated into the digital modulation signals Smod1 and Smod2. These modulation signals are converted into electrical analog modulation signals that are amplified by a respective amplifier 11, 13. The amplifiers may be linear or non-linear amplifiers, wherein a predetermined non-linearity may be used to optimize the creation of exact I- or Q-values (or of I- or Q-values with a sufficient accuracy) in case the corresponding digital-to-analog converter 7, 9 is unable to output a corresponding exact or sufficiently accurate digital value due to a rather low resolution.
The amplified electrical analog modulation signals are fed to the modulation signal input of an optical Mach-Zehnder modulator 17, 19 provided in the I branch and Q branch of an optical IQ modulator 21. The optical IQ modulator 21 receives a coherent optical carrier signal Sc provided by a coherent continuous wave light source 23, for example a laser source, at a splitting point 25 realized by, for example, an optical 1:2 splitter. Between this splitting point 25 and a combination point 27, the I branch and Q branch of the optical modulator extend, wherein in the I branch the first Mach-Zehnder modulator 17 and in the Q branch the second Mach-Zehnder modulator 19 and a phase shifting device 29 are provided. The phase shifting device 29 may be provided before or after the second Mach-Zehnder modulator 19 in the direction of the signal flow.
The splitted signal Sc in the I branch is amplitude-modulated by the first Mach-Zehnder modulator 17 according to the amplified electrical analog modulation signals output by the amplifier 11 and fed to the electrical modulation input port of the modulator 17. Similarly, the splitted signal Sc in the Q branch is phase-shifted by the phase shifting device 29 by a fixed predetermined value of 90 degrees and amplitude-modulated by the second Mach-Zehnder modulator 19 according to the amplified electrical analog modulation signals output by the amplifier 13 and fed to the electrical modulation input port of the modulator 19. The modulated I branch signal and phase-shifted and modulated Q branch signal are combined at the combination point 27, the combination being realized as an adding of the two signals. The combined modulated transmission signal Stx,mod is provided at the output port 15 of the modulating device 1.
The signal Stx,mod may then be transmitted over an optical transmission link and received at the remote end of this link. The received signal may be demodulated via an optical IQ demodulator.
As the optimum constellation points are not located on a rectangular grid, the required DAC resolution to achieve exact constellation points or to achieve constellation points with a sufficiently high accuracy is high. FIG. 2 shows, as an example, the optimum constellation scheme for 7 points based on the “simplex” scheme. For the I-branch, 5 discrete values are required (−1,0; −0,5; 0; 0,5; 1,0) whereas for the Q-branch 3 values (−0,866; 0; 0,866) are required. The minimum resolution is therefore 3 bits. In addition, the amplitudes for the two branches differ, which leads to specific implementation challenges.
In order to reduce the number of discrete values or states of the electrical analog modulation signals, DE 20 2006 000 197 U1 describes an optical IQ transmitting device using a phase modulator in series with the respective Mach-Zehnder modulator in the I and Q branch, the phase modulator receiving control signals in order to provide for a phase shift of 0 or 180 degrees, respectively. In this way, two constellation points axially symmetric to the I or Q axis can be realized (through generating a corresponding modulated signal to be transmitted) by using the same value for the respective modulation signal fed to the Mach-Zehnder modulator and determining the algebraic sign of the constellation point by controlling the additional phase shift generated by the additional phase modulators to be 0 degrees or 180 degrees, respectively.
However, the disadvantage of this optical IQ transmitting device is that the number of discrete values necessary to generate the modulation signals according to the given constellation points is reduced only if the modulation scheme consists of as many pairs of axial symmetric constellation points as possible. Moreover, this structure of an optical IQ modulator requires additional hardware for realizing controlling the status of the additional phase modulators. Finally, the required resolution of the DACs may still be high if the discrete values of the modulation signals to be generated do not match with the discrete values creatable with a DAC having a lower resolution.